The top U.S. official to six party talks on North Korea's nuclear program says the agreement reached last week in Beijing is a good first step. But he warned North Korea that it would have to answer to four other nations if it tries to, in his words, "walk away" from further negotiations.

Ambassador Chris Hill called last week's agreement between the United States, North Korea, China, South Korea, Japan and Russia important, but said it is just the beginning.

"What we did in Beijing is come up with a set of principles that will guide our way through the next and most difficult, perilous, part of the journey," said Mr. Hill.

He said these principles emphasize denuclearization of the Korean peninsula and Pyongyang's return to the Nuclear Non-Proliferation Treaty, which means North Korea would be subject to international inspections and safeguards.

One sticking point, though, is the issue of energy-generating light-water nuclear reactors, which the agreement does mention. North Korea wants them right away. Ambassador Hill said the agreement states that the six parties can discuss the reactors at "an appropriate time," which he says is after North Korea's denuclearization is verified. He added that the issue of verification is one of the subjects for the next round of talks.

"If the North Koreans have truly made a strategic decision to get rid of their weapons, what we envision is a cooperative approach, where we will be assisted, we, the international community, will be assisted by the North Koreans in the verification," said Mr. Hill. "And we do not look forward to, as I've said before, to having some sort of Easter egg hunt or some sort of game of hide and seek."

The U.S. official added his hope that an ultimate six party accord on the North Korean nuclear crisis will also help with efforts to replace the armistice agreement that ended the Korean War with a more formal peace treaty. The Korean War lasted from 1950 to 1953.

"And I think we ought to look and see if we can take this six party accord, or what could become a six party accord, and see if we can use the momentum of that to create some momentum to get us beyond what is essentially a ceasefire armistice, toward some kind of peace mechanism," he added.

The next round of talks is scheduled to be held in Beijing in November. Mr. Hill said his schedule between now and then has not yet been finalized, but he indicated that the United States wants to use its time as productively as possible.